Changes in atmospheric eddy length with the seasonal cycle and global warming

Abstract

A recent article by Kidston et al. [8] demonstrates that the length of atmospheric eddies increases in simulations of future global warming. This thesis expands on Kidston et al.'s work with additional studies of eddy length in the NCEP2 reanalysis (a model-data synthesis that reconstructs past atmospheric circulation) and general circulation models (GCMs) from the Coupled Model Intercomparison Project phase 3. Eddy lengths are compared to computed values of the Rossby radius and the Rhines scale, which have been hypothesized to set the eddy length. The GCMs reproduce the seasonal variation in the eddy lengths seen in the reanalysis. To explore the effect of latent heating on the eddies, a modification to the static stability is used to calculate an effective Rossby radius. The effective Rossby radius is an improvement over the traditional dry Rossby radius in predicting the seasonal cycle of northern hemisphere eddy length, if the height scale used for calculation of the Rossby radius is the depth of the free troposphere. There is no improvement if the scale height is used instead of the free troposphere depth. However, both Rossby radii and the Rhines scale fail to explain the weaker seasonal cycle in southern hemisphere eddy length. In agreement with Kidson et al., the GCMs robustly project an increase in eddy length as the climate warms. The Rossby radii and Rhines scale are also generally projected to increase. Although it is not possible to state with confidence what process ultimately controls atmospheric eddy lengths, taken as a whole the results of this study increase confidence in the projection of future increases in eddy length.